Do I need a bigger inverter?

Redlander

I'm intending to get solar panels in the near future. An installer has proposed:
8 panels totalling 4.08 kWp
DC-coupled battery 10.36 kWh
Inverter 3.68 kW

Eventually our house will have:
Air source heat pump 5 kW
Electric oven
Induction hob
(No intention of getting an EV)

Clearly the inverter is adequate for the solar panels, but I'm hoping to use the battery for various flexible Octopus tariffs, so I'm thinking that the inverter will restrict the ability of the battery to discharge.

So should I upgrade the inverter to, say 5 kW? (I'm aware of the more stringent DNO approval conditions)

zxzxzx

4.8 solar, 3.6 inv, 2x5.2 batteries installed about 15 months ago.

If I knew then etc. max out panels - fill your roof/rooves, get the biggest inverter to match (this impacts on how much electric can be fed in/out) and spend whatever money you have left on batteries.

If you are considering an EV then think about vehicle to grid/home as you battery store in the future.

Screwdriva

Just 5 questions before I can help:

1) Whereabouts is the roof located?
2) South facing or East/West aspects?
3) Any reason why you’ve stated only 8 panels?
4) What is the timeframe for getting the ASHP?
5) What battery brand has the installer who has quoted you specified?

Be back with more thoughts once I learn more! 

QrizB

Based on the details you've given, I think your inverter is big enough. It's a good match to your solar panels and is still big enough to dump a full battery in 3 hours.

Redlander

Screwdriva said:

Just 5 questions before I can help:

1) Whereabouts is the roof located?
2) South facing or East/West aspects?
3) Any reason why you’ve stated only 8 panels?
4) What is the timeframe for getting the ASHP?
5) What battery brand has the installer who has quoted you specified?

Be back with more thoughts once I learn more! 

1. It's a mid-terrace house
2. The roof faces south-south-east
3. eight seems to be the maximum that will fit between mine and my neighbour's chimney stack
4. Week beginning March 24th
5. FoxESS

Screwdriva

Redlander said:

1. It's a mid-terrace house
2. The roof faces south-south-east
3. eight seems to be the maximum that will fit between mine and my neighbour's chimney stack
4. Week beginning March 24th
5. FoxESS

Your inverter size is perfectly matched to the roof and the SSE aspect is the best performing of all UK aspects, so that's a win. That said, your battery is undersized given your ASHP requirement - the minimum size I'd recommend is 13.5kW. I'm not a fan of Fox products and find the software, integration and aftersales to be quite poor. Cold weather performance is abysmal.  

In my view, the best battery on the market is the Powerwall 3. It has unmatched cold weather performance, safety & reliability track record, aftersales and warranty support, software and app. 

I'm not sure what you've been quoted? but a Powerwall 3 paired to 8 on roof panels should cost no more than £10.5K installed. Best of all, the inverter is built into the PW3, so there's no need for a separate inverter. Just one aesthetically pleasing unit.

Martyn1981

Don't take this too seriously, it's just an option, but how about wall mounted panels too? They will generate about 70-75% of roof pitch panels, but work well in the winter*, when you will appreciate every last drop of generation.

I think the Powerwall 3 has 3 MPPT inputs, so that would work well. And upto 11kW supply. I assume (sorry don't know) that output or export can be limited to meet DNO approval.

*OK, 'well' may be confusing, no PV in the UK works well in the winter, but steep/vertical pitch panels generate more in the winter than they would at say 35d pitch. Double check, but I understand that PP is no longer needed for wall mounted PV.

ed110220

Redlander said:

I'm intending to get solar panels in the near future. An installer has proposed:
8 panels totalling 4.08 kWp
DC-coupled battery 10.36 kWh
Inverter 3.68 kW

Eventually our house will have:
Air source heat pump 5 kW
Electric oven
Induction hob
(No intention of getting an EV)

Clearly the inverter is adequate for the solar panels, but I'm hoping to use the battery for various flexible Octopus tariffs, so I'm thinking that the inverter will restrict the ability of the battery to discharge.

So should I upgrade the inverter to, say 5 kW? (I'm aware of the more stringent DNO approval conditions)

Based on my experience I'd definitely go for a larger inverter. It's also worth noting that the size of the inverter is usually given as what it can output in AC from the PV, but the charge/discharge power from batteries can be smaller than this. So that's worth checking.

I've got a Growatt SPH5000 which is a 5 kW inverter, but it's maximum charge/discharge rate is 3 kW. Paired with 4.8 kW of PV and a 15 kWh battery.

For the battery and inverter I'd look at:

What the charge/discharge power required is. We can get by with 3 kW without importing much from the grid, but it does mean not running high power items at the same time. If you have ASHP you'll often be drawing more than 3 kW I would expect, so if you were say drawing 5 kW with an inverter like mine 2 kW would be coming from the grid even if your battery is full.

How much electricity you'll use in 24 h in winter. With a battery you'll really want to be on a tariff that offers cheaper off peak electricity, at least in winter. You'll also want an inverter that is capable of charging your battery up with that amount overnight. With my 3 kW I can charge up with 15 kWh in the 5 hour Octopus Go cheap rate period which is enough for us, so our average import cost is quite close to 8.55p. If we were on Flux which only offers 3 hours it would only be 9 kWh which wouldn't be enough many days. Again, I expect you'll be using significantly more with ASHP.

If I were starting from scratch and looking to get ASHP I'd be looking at at least a charge/discharge power of 6 kW, which would be capable of charging up 30 kWh in that 5 hour overnight window. Even if I wasn't going to get 30 kWh of batteries straight away because batteries will probably carry on falling in price and are relatively easy to install more of later.